1. Field of the Invention
The present invention relates to a broad-band non-grounded type ultrashort-wave antenna which is desirable as, for example, a wireless telephone antenna used in an automobile telephone system, etc.
2. Prior Art
In recent years, there has been a rapid expansion in the development and utilization of the so-called "mobile communication systems" such as automobile wireless telephone systems, and even more recently, there has been a technological shift toward a high degree of digitalization. As for the frequency band used in automobile wireless telephone systems, new frequency bands have been added to both ends of the currently used frequency band in order to achieve a wider diffusion of digital systems while using such digital systems along the conventional analog systems.
At NTT (Nippon Telegram and Telephone Corporation in Japan), for example, a band that extends from 865 MHz to 945 MHz (i.e., a band with a width of 80 MHz) was used in the past; but with a rapid development of digitalization, this has been changed to a band width of 810 MHz to 960 MHz (i.e., a band with a width of 150 MHz). In other words, the band width presently used is approximately twice the conventional band width.
One of the antennas used in the past has an electrical length set at lambda/2 (.lambda./2), and another uses a so-called "constant-K filter." These antennas have satisfactory sensitivity characteristics and impedance characteristics (especially the VSWR characteristics), both required for mobile communications for the conventional 80 MHz band width. However, with respect to the new band width of 150 MHz which is approximately double the old band width as described above, the conventional antenna is unsatisfactory for either one or both of the sensitivity and impedance characteristics (especially the VSWR characteristics).
More specifically, in the antenna which has an electrical length of lambda/2, the sensitivity characteristics are good, but the impedance characteristics, especially the VSWR characteristics, are more or less unsatisfactory. On the other hand, in the antenna which uses the constant K filter, the impedance characteristics, especially the VSWR characteristics, are more or less good but the sensitivity characteristics are unsatisfactory.
As seen from the above, though the band width of the frequency band used for automobile wireless telephone systems has been approximately doubled due to the digitalized communications systems, the conventional antennas cannot satisfy the sensitivity characteristics nor the impedance characteristics, especially the VSWR characteristics.
One way to improve the VSWR characteristics or to achieve the broad band characteristics is to enlarge the diameter of the antenna element so as to reduce the inductance of the antenna element and to increase the capacitance, thus lowering the Q value of the antenna. However, in the automobile antennas, the wind pressure resistance increases as the diameter of the antenna element becomes larger. Accordingly, to increase the antenna diameter is not desirable from the design standpoint. Thus, there are inherent limits in the effort to increase the diameter of the antenna elements.
Another method to achieve the broad band characteristics is to incorporate lambda/4 matching devices into a multiple number of stages of the antenna element. This method, however, requires that the antenna itself be made to have broad band characteristics. Ordinarily, therefore, broad band characteristics are realized by a combination of a use of lambda/4 matching devices and a use of enlarged diameter antenna elements. However, in this combination, the structure tends to be complex and a high degree of technical skill is required to build the antenna. Furthermore, the cost of the antenna rises and the weight of the antenna increases.